Light-emitting semiconductor devices have been developed over the past several decades to emit either incoherent light (devices referred to as “LED's”) or coherent light (referred to as “laser diodes” or “diode lasers”.) A common attribute of both LED's and laser diodes is a pair of electrically conducting surfaces, serving as the anode and cathode of the device, through which an electrical current is passed to generate light. In devices where a high optical output is desirable (e.g., in some LED lighting applications and especially in high-power laser diodes), a critical aspect of the device mounting relates to the ability of such mounting to effectively remove waste heat from the device. This is of particular importance where high power, high efficiency and high reliability are desired, since all light-emitting semiconductor devices exhibit declining electrical-to-optical efficiency as the diode temperature increases; and ultimately succumb to catastrophic failure if the diode temperature continues to rise, either with prolonged emission or at higher current levels.
Both LED's and laser diodes are of remarkably small size given the amount of light these devices can emit, the amount of power they consume, and the amount of heat that they generate. By way of example, a single laser diode “chip” emitting laser light at 1470 nm (Seminex Corporation, Peabody, Mass.) can readily produce four watts of continuous-wave laser emission with an input current of ten amperes and a voltage drop of about two volts (an input electrical power of 20 watts, resulting in an efficiency of 20%). The dimensions of this chip are only 0.5 mm wide by 2.5 mm in length, with a thickness of 0.16 mm. Even more remarkably, the actual laser light is emitted by a very thin, narrow region centered on the anode surface (the so-called “P-side”) that is only 95 microns (0.095 mm) wide and 1 micron in height. This “laser stripe” on the P-side has a surface area of 0.095 mm by 2.5 mm, or 0.24 mm2. Since in the above example 80% of the input electrical power becomes waste heat, the power density on the laser stripe surface exceeds an enormous 6,000 watts per square centimeter. This represents a significant amount of thermal energy that must be efficiently conducted away from the laser diode to avoid overheating or catastrophic failure.
To maximize the conduction of heat away from a hot surface, the so-called thermal conductivity between the hot surface and the cooler contacting surface must be maximized. Two approaches have generally been used: either mechanically pressing the heatsink against the hot surface (often with an interface substance such as “thermal grease” placed in between the two surfaces); or by bonding the two surfaces together with a solder, or glue such as a high-thermal conductivity epoxy. The mechanical approach has not proved practical with semiconductor devices such as laser diodes, for two reasons. First, the laser itself has no casing or housing, and the laser material is quite brittle. Thus, the pressure necessary to achieve good thermal conductivity using a hard heat sink surface can result in cracking of the semiconductor device, either immediately or during use. Secondly, while thermal grease can help achieve high thermal conductivity with a lighter pressure, the grease tends to migrate toward the optical surfaces of the laser diode, resulting in permanent damage to the laser.
Solder bonding has thus been used more frequently for semiconductor devices such as laser diodes, since it offers good thermal conduction to a heatsink, typically better than an epoxy, without putting direct mechanical pressure on the brittle diode material. However, this approach has its own inherent drawbacks, especially for laser diodes, which are typically fabricated out of gallium arsenide or indium phosphide. A simple soldering to a copper heatsink routinely fails due to the difference in the thermal expansion coefficient between the laser diode material and the heatsink material. To overcome this problem, more difficult to fabricate (and expensive) heatsink material must be used, having a comparable thermal expansion coefficient. Because the thermal expansion coefficient of solder is also not perfectly matched to that of the semiconductor, it too places stresses on the laser diode.
A related challenge for soldering is that the laser diode has an output facet coating consisting of a partially-transmitting, multilayer dielectric material, and a highly reflective coating on the rear laser facet. Both of these optical surfaces must be completely undisturbed during the soldering process, or the laser will likely fail. Thus, many fluxes that might typically be used to remove oxides during the soldering process cannot be used.
In addition, it is desirable to subject laser diodes to minimal temperature rise during soldering, but low melting-point solders such as indium and its alloys do not generally “wet” surfaces as well as higher temperature solders. This requires that the laser diode surface be designed to be as “solderable” as possible. Thus the metallization on the P-side is typically a “sandwich” of vacuum-deposited layers consisting of for example, a 50-nanometer layer of titanium, followed by a 125-nm layer of platinum, and covered finally with a 250-nm layer of gold.
Still further, the solder must be in intimate contact with the entire length of the laser stripe; if it is not, the region without contact becomes catastrophically hot. Unfortunately, when the optical coating is deposited onto the output facet of the laser diode chip, some of the coating material may get inadvertently deposited on the P-side of the chip and prevent the solder from bonding to the chip in that area. This typically results in infant failure of the chip. A common reason for rejecting laser chips prior to solder-mounting to a heat sink is such facet coating “overspray.”
Lastly, as a final additional complexity of some commercially-mounted laser diodes, as many as 6 to 12 microscopic gold wires are individually ball-bonded onto the cathode surface of the laser diode, for the purpose of providing a return current path from the laser device.
From the foregoing, it will be appreciated that there has long been a need for a method of mounting semiconductor devices such as laser diodes that provides the desired thermal and optical performance while being simpler and more reliable to manufacture and assemble.